Antenna arrays may be used in any wireless communication receiver, transmitter or transceiver that transmits or receives radio frequency signals using an antenna. The use of antenna arrays provides for performance improvements over conventional devices that communicate using a single antenna element. These improvements include but are not limited to improvements in the signal-to-noise ratio (SNR) and the signal-to-interference-plus-noise ratio (SINR) of received and transmitted signals, and improvements in the directionality with which signals can be received from or transmitted to a selected device.
An example of a wireless communication system that includes an antenna array is a cellular communication system consisting of one or more base stations, each communicating with its subscriber units, also called remote terminals or handsets. In cellular systems, the remote terminal may be mobile or in a fixed location. Antenna arrays can be used at the base station transceiver, at the handset transceiver, or at both locations to improve the communication link between the devices. Antenna arrays can be used in a wireless communication system to add Spatial Division Multiple Access (SDMA) capability to the system. SDMA refers to the ability to spatially multiplex a conventional communication channel such as a frequency band in a Frequency Division Multiple Access (FDMA) system, a time slot in a Time Division Multiple Access (TDMA) system, or a pseudo-random code in a Code Division Multiple Access (CDMA) system.
Adaptive smart antenna processing may be used in a transceiver equipped with multiple antennas, and involves developing a beam-forming strategy to either reject interference when receiving a signal from a selected transceiver (an uplink strategy), or to deliver power in a spatially or spatio-temporally selective manner when transmitting a signal to a selected transceiver (a downlink strategy). In a linear system, developing an uplink strategy involves finding a series of complex weights in a receive weight vector such that the inner product of the receive weight vector and the signals received at each of the antenna array elements preferentially selects or enhances the signals transmitted by a selected transceiver while preferentially rejecting or minimizing spurious signals transmitted by one or more noise or interference sources.
The contents of the receive weight vector for a selected transceiver can be determined from knowledge about the signals transmitted by that transceiver such as the data content of the signals, or the way in which data is modulated onto the signals. For example, a transmitted signal can be reconstructed from a plurality of signals received by an antenna array by adjusting the weights in a variable receive weight vector so that the reconstructed transmitted signal has a constant modulus. This method of determining the receive weight vector is useful in communication systems that use a modulation scheme in which information is encoded onto a constant modulus carrier signal. Examples include phase modulation (PM), frequency modulation (FM), phase shift keying (PSK) and frequency shift keying (FSK). Other methods of determining the receive weight vector are also possible. For example, a transmitted signal can be reconstructed from a plurality of signals received by an antenna array by adjusting the weights in a variable receive weight vector so that the data content of the reconstructed transmitted signal agrees with locally generated training data. Training data are data symbols that are known to have been sent by a selected transceiver at a known time in a signal burst from the selected transceiver.
Similar to developing an uplink strategy, developing a downlink strategy in a linear system involves finding a series of transmit weights in a transmit weight vector such that when a signal to be transmitted by an antenna in the antenna array is weighted by its corresponding transmit weight from the transmit weight vector, the net signal transmitted by all of the antennas in the antenna array is preferentially directed toward a selected transceiver while being preferentially directed away from one or more sources of noise or interference. In some systems, for example in time-division duplex (“TDD”) systems, the transmit weight vector may be determined in part from the receive weight vector.
The development of beam-forming strategies in an adaptive smart antenna processing systems can be based on a number of competing factors. Such factors include but not are limited to the quality of signals received from or transmitted to a selected transceiver, the absolute power delivered to a selected transceiver, the relative importance between a selected transceiver and one or more interfering transceivers, the relative need for mitigation or nulling of the signals received from or transmitted to an interfering transceiver, the input signal-to-noise (SNR) ratio, the carrier-to-interference ratio (CIR), the bit error rate (BER) and the spatial correlation between a selected transceiver and an interfering transceiver. Since it is impossible to develop a beam-forming strategy that simultaneously optimizes each of these competing factors, there is a need for smart adaptive antenna processing systems that can adaptively develop different beam-forming strategies to operate efficiently in different situations.